Danish researchers have perfected an inexpensive and efficient way to convert types A, B and AB blood into type O, the universal-donor blood that can be given to anyone — an achievement that promises to make transfusions safer and relieve shortages of type O blood.

The team reported Sunday in the journal Nature Biotechnology that they isolated bacterial enzymes that safely remove from red blood cells the sugar molecules that provoke an immune reaction in the recipient.

Previous studies of type O blood produced from type B by a different method have shown it to be both safe and effective, and the researchers are now conducting clinical trials with the new product.

Mismatching of blood causes at least half of all transfusion-related deaths. And the need for precisely matched blood drives the costly and inefficient process of shuttling blood units between regional blood banks and hospitals to match daily requirements.

“Those issues could be largely resolved if there were a universally transfusible blood supply,” said Doug Clibourn, chief executive of ZymeQuest Inc. in Beverly, Mass., which is developing the technology.

The problem involves sugar molecules on the surface of red blood cells. Type A blood has one set of sugars and type B has another, while type O has none. People with type A blood have antibodies against the type B sugars, people with type B have antibodies against type A, and people with type O have antibodies against both.

In the 1980s, the late Jack Goldstein of the New York Blood Center isolated an enzyme from coffee beans that could convert type B to type O. Clinical trials of the enzyme-produced blood showed it behaved no differently from normal blood in hospitalized patients.

But the enzymes involved were expensive and had to be used under highly acidic conditions that damaged the red cells. Goldstein”s team also was not able to find an enzyme that would convert type A to type O.

As a consequence, the development was halted.

ZymeQuest commissioned cellular biologist Henrik Clausen of the University of Copenhagen to search for new enzymes to carry out the conversion. Clausen and his team sifted through more than 2,500 bacteria and fungi before identifying the two candidates cited in the Nature Biotechnology report.

The discovery could be a major breakthrough in improving the blood supply, wrote Geoff Daniels of England”s Bristol Institute for Transfusion Sciences in an editorial accompanying the article.

The new enzymes are more potent than previously used ones and, more important, they work at room temperature and neutral pH, which is very good for blood cells, said Dr. Martin L. Olsson of Lund University in Sweden. In an hour, they remove all the sugar molecules from the surface of red blood cells, after which they can be easily washed away.

The team initially isolated blood from healthy individuals, converted the red cells to type O and injected them back into the donors, said Olsson, who is overseeing the clinical trials.

After that study showed no problems, they began a larger clinical trial using donor blood. Olsson refused to comment on the results but confirmed that there have been no adverse reactions to the product.

Clibourn said he expects results from the trial to be available later this year.